This invention relates to cable testers. More particularly, the invention relates to cable testers for multi-pair shielded cables.
A multi-pair shielded cable is a cable made up of a plurality of pairs of insulated wires disposed inside a sleeve of insulating material, the wires of each pair being twisted about each other and each pair of wires being enclosed within a separate grounding shield. Multi-pair shielded cable is used extensively in the communications and computer industries.
For example, in most all computerized cash register systems that are now in use or are being installed for use in department stores, each one of the cash registers is connected to a main junction box by means of a multi-pair shielded cable. Normally, the wires at one end of the cable are directly connected to appropriate terminals on the main junction box whereas the wires at the other end of the cable are connected to appropriate terminals on a socket which is designed to mate with a plug in the cash register. The number of pairs of wires in the cable is dependent on the type of system being used. For most cash register systems, the number of pairs of wires per cable is generally between two and five. For other systems and machines other than cash registers the number of pairs of wires per cable can be even greater than five. In order for the system to operate properly it is essential that the cable must be connected properly at each end. In addition, the cable must contain no electrical defects. Some of the typical faults that may be present either prior to installation or during the course of use are as follows:
1. shorts between one or more wires and the grounding shield; PA1 2. shorts between two wires of a pair; PA1 3. lack of continuity between the two wires of a pair; PA1 4. incorrect pair identification (i.e., the pairs of wires at one end of the cable are connected to a wrong set of terminals); and PA1 5. incorrect polarity of a pair of wires (i.e., the wires of one pair are connected at one end of the cable to its termination in reverse order).
Hitherto, testing for faults (1) through (4) above has been accomplished by using either an ohm-meter, a buzzer or a flash light along with a shunting element. Testing for fault (5) above has been accomplished by using either an ohm-meter, a buzzer or a flash light and shunting element in combination with a diode. In either case the testing procedure using such devices has proven to be very time consuming since the particular device that is used has to be connected separately to each pair of wires in the cable and then to each wire in the cable and the grounding shield. In addition, the procedure is subject to much human error on the part of the person performing the test.
In U.S. Pat. No. 3,944,914 is disclosed an apparatus for performing a variety of electrical tests on a multiconductor (but not multi-pair shielded) cable, one pair at a time and in U.S. Pat. No. 3,931,574 is disclosed a test module containing light emitting diodes for testing integrated circuit connections. Examples of other test devices pertinent in one way or another to this invention can be found in U.S. Pat. Nos.. 3,333,188; 3,600,678; 3,806,803; 3,826,977; 3,831,089; 3,860,868; 3,904,958, and 3,962,630.
As used throughout this specification, the term "termination" means any device having terminals, such as a plug, a socket, a jack, a connector, or a junction to which can be attached a plurality of wires.